Here is a full briefing I have for users of ISPConfig:
1. Create a new zone for the block of IPs and assign the new zone to the client
Example: Customer has 192.168.10.8/29, create zone 8-29.10.168.192.in-addr.arpa
(Note: the "8-29" zone prefix is arbitrary but helps us with identification of subnet)
2. Move the records (Example: hosts 8 through 15) manually or via database to new zone
3. In old zone (10.168.192.in-addr.arpa), add NS records for the new zone prefix
Example: NS 8-29 dns1.example.com.
NS 8-29 dns2.example.com.
4. In old zone, add CNAME records for each host in customer's range pointing to the new zone
in the form of CNAME [host] [host].[new zone prefix]
Example: CNAME 9 9.8-29
CNAME 10 10.8-29
5. Refresh the zones.
Now a PTR lookup will for 192.168.10.11 will query 11.10.168.192.in-addr.arpa and get a CNAME
of 11.8-29.10.168.192.in-addr.arpa which will return the proper value for host 11 in the newly
created zone.

(Note: This procedure can be applied to a single record and non-continous ranges)